The invention relates to improved methods of forming a feedstock, comprising a wax-impregnated coal, for coal gasification.
The gasification of solid fuels such as coal is well known. Several methods have been proposed for feeding the coal into the gasifier. In one method, the coal is ground to a fine powder and fed to the gas generator as a suspension in steam or a free oxygen-containing gas. This method is unsatisfactory as it is difficult to control the amount and rate of the coal fed to the gas generator and, in the case of a free oxygen-containing gas, care must be taken to maintain the velocity of the suspension above the rate of flame propagation to avoid a dangerous and damaging backflash.
Newer methods have been developed to overcome the drawbacks of the dry, ground coal feedstock. One method is the production of a coal-water slurry in which the coal is ground to a particle size, mixed in water or organic liquids, and injected into the gasifier. The coal is ground to a fine particle size to ensure that almost complete conversion of carbon to oxides takes place during the residence time in the gasification zone of the gasifier. To properly feed such a slurry into the gasification zone, the slurry must be conveyed from the point at which it is generated to the gasifier. The slurry must not be too viscous to be pumped from its starting point to its destination but, simultaneously, cannot be diluted to a level that will cause incomplete or inefficient conversion to gas in a gasifier. The total water content of the slurry must therefore be kept, preferably, close to 30-40%.
This restriction on the water content of a coal-water slurry is readily attained by using high-rank solid coal sources such as anthracite and bituminous coal. However, many coal sources contain varying amounts of inherent water, and in many instances the water content may be as high as 30 weight percent; it may be higher in the case of lower rank coals such as sub-bituminous coal, lignite, and brown coal. The water is present as surface water on the face of the coal, as inherent water found in the smaller pores of the coal, and as chemically bound water within the carbon lattice. This higher water content has made these fuel sources largely useless for the production of a slurry feedstock for a gasifier.
Different approaches have been taken to render these low-rank coals useful as a feedstock for the coal gasification process. For example, the coal may be dried at an elevated temperature. The drying process successfully removes surface and inherent water but is typically incomplete, or too energy intensive, to economically remove chemically bound water in the low-rank coals. Moreover, such dried coals, when formed into a slurry, tend to take up a significant amount of water from the slurry. Mixing of lower-rank coals with a smaller percentage of finely ground, higher-rank coals has also been used to make a less costly fuel although the improvement in cost is minor after providing the means necessary to precisely grind and mix the higher- and lower-rank coals.
Other methods have focused on various chemical treatments to decrease the water content of the coal slurry and thereby boost the (British thermal unit) value of the slurry. Chemicals such as surfactants, detergents, suspension stabilizers, and amines have been used as additives to the slurry to decrease the viscosity, thereby lowering the water content necessary to maintain the pumpability. This effectively increases the concentration of the coal in the slurry, thereby raising the Btu value. Unfortunately, the addition of these chemicals is often expensive and the chemicals themselves can further decrease the efficiency of the gasification process.
U.S. Pat. No. 3,996,026 teaches a method of using organic liquids as additives to the coal-water slurry, which can then be successfully pumped from the source of the slurry to the gasifier. Immediately prior to entering the gasifier, the slurry is fed through a separator where the organic liquids are removed and the coal-water mixture is injected into the gasification zone. In this method, the coal is ground and mixed with water to form a slurry having a water content between 35 and 55% by weight. An organic liquid such as kerosene, hexane, or light vacuum gas oil is then added to the coal-water slurry to improve the pumpability. These chemicals have the added advantage of increasing the Btu value of the lower-rank coals. Unfortunately, these organic liquids are quite valuable and must be recovered, to the extent possible, before the slurry enters the gasifier. For this reason, the slurry is pumped through the machinery to a modified gasifier having a distillation apparatus that recovers the expensive organic chemicals from the slurry before the slurry is added to the gasifier. The organic liquids are removed from the slurry as a super-critical liquid or dense gas and recycled to once again act as an aid to the pumping of the coal-water mixture. The method is limited to organic liquids ranging from four to twenty carbons in length so that they can be successfully removed in the separator before the coal is injected into the gasifier. The method suffers from the greatly increased costs of running the distillation apparatus to recover the expensive organic liquid from the coal-water slurry, as well as the costs associated with continual losses of the expensive organics resulting from incomplete removal from the coal.
Therefore, there still exists a need for an improved method of preparing a feedstock for a coal gasifier that allows the use of lower-rank coals in an economically feasible manner. The method should result in a feedstock having a sufficient Btu value and a restricted water content to ensure economically efficient conversion to a synthesis gas. Preferably, the feedstock should be capable of being conveyed to the gasifier by a means that allows control over the amount and rate of solid fuel entering the gas generator while avoiding potential backflash.
The present invention is a method of beneficiating low-rank coal to produce a relatively high-energy, cohesive, low-moisture, stable feedstock for coal gasification. One embodiment of the invention comprises contacting partially or completely dried low-rank coal with wax at defined temperatures and pressures, thereby forming a wax-impregnated coal. The wax-impregnated coal may be either slurried or formed into briquettes for coal gasification. Gasification produces synthesis gas that can be used to co-produce electricity and liquified Fischer-Tropsch products, including diesel fuel, naptha, and wax. A fraction of the wax can then be recycled to the coal preparation section to aid in materials handling, agglomeration, reducing moisture levels, and increasing the specific energy of the feedstock operation.